Brush seals of the above mentioned general type are often used in fluid flow machines, and especially gas turbine engines, to provide a seal with the minimum possible leakage at circumferential gaps, for example between a machine housing and the rotor or around a rotary shaft of the turbine machine, between two spaces having different prevailing fluid pressures within the turbine machine. Generally, the bristles of the brush seal are held and guided so as to be elastically yieldable or movable in order to compensate for any eccentric or oscillating motions of the rotor or shaft relative to the housing, which may result, for example, from rotor imbalances or the like. In this context, brush seals have been recognized as advantageous over the previously long utilized labyrinth seals, because such labyrinth seals can only adapt themselves to eccentric shaft movements to a very limited degree. Namely, in that context, any forseeable eccentricities are to be taken into account by making corresponding provisions in the gap width. Furthermore, brush seals advantageously distinguish themselves over prior labyrinth seals by a lower production cost and a lower weight.
European Patent 0,214,192 discloses brush seal arrangements of the above mentioned general type, in which the seal bristles may be tilted at an angle .alpha. in the direction of rotation of the rotor or shaft relative to a radial direction, and in which the seal bristles may be adjustably tiltable at an angle .beta. in an axial direction relative to a plane perpendicular to the axis of the rotor or shaft.
U.S. Pat. No. 5,029,875 (Spain et al.) and U.S. Pat. No. 5,474,306 (Bagepalli et al.) disclose various brush seal arrangements in which the bristles may be tilted in a circumferential direction or in an axial direction relative to the annular plane of the bristles. In order to improve the seal effectiveness, Bagepalli et al. suggest to guide the seal bristles between two spaced apart ring or disk-shaped flanks.
The typical resilient deflectability of brush seals relative to eccentric motions of the rotor is achieved by means of the tilted orientation of the bristles of the brush seal relative to the seal surface of the rotor or relative to the rotor radial directions. The required degree of deflectability or elasticity of the brush seal can be influenced by properly selecting the bristle length and the degree of tilting of the bristles, independently of the selection of the material for the bristles.
Especially in the above described embodiments having the bristles tilted in the circumferential direction, it has been discovered in practice that the brush seal is sensitive to the rotational direction of the rotor. Namely, if the rotor rotates in a reverse direction, then the bristles, which are tilted in a direction against or opposite such a reverse rotor rotation, become jammed against the rotor seal surface and are thus pushed along with the rotor and consequently become bent in a direction opposite the original tilt angle. As a result, the bristles may be permanently plastically deformed, such that gaps result in the brush seal. This leads to a diminishing seal effectiveness of the brush seal.